I just tried

emerge -DuN @world @system -va

and got:

These are the packages that would be merged, in order:

Calculating dependencies ... done!
[ebuild     U  ] perl-core/ExtUtils-ParseXS-3.20.0 [2.22.06] 0 kB
[ebuild     U  ] www-client/firefox-bin-5.0 [3.6.19]
USE="startup-notification"
[ebuild  N     ] x11-themes/tango-icon-theme-0.8.90  USE="png" 0 kB
[ebuild  N     ] media-plugins/kipi-plugins-2.0.0  USE="cdr crypt
imagemagick mediawiki opengl (-aqua) -calendar -debug -expoblending
-gpssync -handbook -ipod (-kdeenablefinal) -mjpeg -redeyes -scanner"

[ebuild  N     ] media-gfx/digikam-2.0.0  USE="gphoto2 handbook
thumbnails -addressbook (-aqua) -debug -doc (-kdeenablefinal) -mysql
-semantic-desktop -themedesigner -video"

[ebuild  N     ] xfce-base/xfce4-meta-4.8  USE="session svg -minimal" 0 kB

Total: 6 packages (2 upgrades, 4 new), Size of downloads: 15,432 kB

The following keyword changes are necessary to proceed:
#required by @selected, required by @world (argument)
>=perl-core/ExtUtils-ParseXS-3.20.0 ~amd64

NOTE: This --autounmask behavior can be disabled by setting
      EMERGE_DEFAULT_OPTS="--autounmask=n" in make.conf

But if I run:

EMERGE_DEFAULT_OPTS="--autounmask=n" emerge -DuN @world @system -va

I get:

These are the packages that would be merged, in order:

Calculating dependencies                   ... done!
[ebuild     U  ] www-client/firefox-bin-5.0 [3.6.19]
USE="startup-notification"
[ebuild  N     ] x11-themes/tango-icon-theme-0.8.90  USE="png" 0 kB
[ebuild  N     ] media-plugins/kipi-plugins-2.0.0  USE="cdr crypt
imagemagick mediawiki opengl (-aqua) -calendar -debug -expoblending
-gpssync -handbook -ipod (-kdeenablefinal) -mjpeg -redeyes -scanner"

[ebuild  N     ] media-gfx/digikam-2.0.0  USE="gphoto2 handbook
thumbnails -addressbook (-aqua) -debug -doc (-kdeenablefinal) -mysql
-semantic-desktop -themedesigner -video"

[ebuild  N     ] xfce-base/xfce4-meta-4.8  USE="session svg -minimal" 0 kB

Total: 5 packages (1 upgrade, 4 new), Size of downloads: 15,432 kB

Would you like to merge these packages? [Yes/No]

[LINGUAS was stripped by me to remove unneded clutter from this post]

I run ~amd64 but have =perl-core/ExtUtils-ParseXS-3.20.0 -~amd64 because
of a bug with imagemagick [#378383].

Why is autounmask trying to unmask ExtUtils-ParseXS if it's not needed?
Should I report this as a bug?

Greetings

Sebastian Beßler